golang/go · error

failed to compute server URL: %v

Error message

failed to compute server URL: %v

What it means

Once the listener is up, `addrURL(addr)` converts the bound address into an `http://...` URL by calling `net.SplitHostPort` again on the listener's resolved address. If the resolved `ln.Addr().String()` cannot be split into host:port (extremely unlikely for a TCP listener), the URL computation fails and the tool aborts before serving. This is a defensive guard around an expected-invariant case.

Source

Thrown at src/cmd/trace/main.go:165

		default:
			logAndDie(fmt.Errorf("invalid debug mode %s, want one of: parsed, wire, footprint", *debugFlag))
		}
	}

	addr, err := listenAddr(*httpFlag)
	if err != nil {
		logAndDie(fmt.Errorf("malformed -http value %q: %v", *httpFlag, err))
	}

	ln, err := net.Listen("tcp", addr)
	if err != nil {
		logAndDie(fmt.Errorf("failed to create server socket: %w", err))
	}

	addr = ln.Addr().String()
	url, simplified, err := addrURL(addr)
	if err != nil {
		logAndDie(fmt.Errorf("failed to compute server URL: %v", err))
	}

	log.Print("Preparing trace for viewer...")
	parsed, err := parseTraceInteractive(tracef, traceSize)
	if err != nil {
		logAndDie(err)
	}
	// N.B. tracef not needed after this point.
	// We might double-close, but that's fine; we ignore the error.
	tracef.Close()

	// Print a nice message for a partial trace.
	if parsed.err != nil {
		log.Printf("Encountered error, but able to proceed. Error: %v", parsed.err)

		lost := parsed.size - parsed.valid
		pct := float64(lost) / float64(parsed.size) * 100
		log.Printf("Lost %.2f%% of the latest trace data due to error (%s of %s)", pct, byteCount(lost), byteCount(parsed.size))

View on GitHub (pinned to b6b368adc5)

Solutions

1. Retry with a plain `-http=localhost:<port>` to ensure a normal TCP listener address.
2. Update/repair the Go toolchain — a malformed listener address indicates a toolchain or platform bug.
3. If running a patched tool, ensure the listener's Addr() returns a valid host:port string.

if _, _, err := net.SplitHostPort(ln.Addr().String()); err != nil {
    log.Printf("listener returned unexpected address format: %v", err)
}

When it happens

Trigger: Effectively only if `net.Listener.Addr().String()` returns a malformed string, which does not happen for standard TCP listeners. Could occur with a custom/intercepted net.Listener in a fork, or on platforms whose net package returns nonstandard address formatting.

Common situations: Rare in stock Go; seen when a third-party wrapper proxies the listener (e.g., a custom dialer/library that fakes listener addresses), or in stripped/odd toolchain builds.

Related errors

• malformed -http value %q: %v
• failed to create server socket: %w
• failed to start http server: %w
• stopped after 10 redirects
• file URL specifies non-local host

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/33d9eb8fc8151e33. Report an issue: GitHub.